The first reference to the preparation of aryltriazoles is Berichte 9, 219 (1876) in which benzotriazoles and tolyltriazoles were prepared. In describing this process for purifying aryltriazoles, we specifically include benzotriazole, tolyltriazole and alkyl-substituted analogs of both. The term tolyltriazole is meant to mean 4-methylbenzotriazole, 5-methylbenzotriazole and mixtures of the two. Non-proprietary processes for the preparation of aryltriazoles were subsequently published in Ber. Chem. 33, 261 (1900), Gazz. Chim. Ital. 51, 267 (1921), J. Chem. Soc. 954 (1926), J. Am. Chem. Soc. 57, 1835 (1935), Organic Synthesis 20, 16 (1940), and Chem. Berichte 100, 1646 (1967). Proprietary references to the preparation of aryltriazoles are U.S. Pat. No. 2,861,078 (1958), U.S. Pat. No. 3,227,726 (1966), U.S. Pat. No. 3,637,514 (1972), DT2,351,595 (1973), U.S. Pat. No. 3,732,239 (1973), JP51-65760 (1976), U.S. Pat. No. 4,158,660 (1979), GB1,581,407 (1980), U.S. Pat. No. 4,299,965 (1981), U.S. Pat. No. 4,363,914 (1982), U.S. Pat. No. 4,424,360 (1984), U.S. Pat. No. 4,528,381 (1985), U.S. Pat. No. 4,549,026 (1985), and U.S. Pat. No. 5,914,409 (1999).
Generally, these methods of preparation produce an aryltriazole product that is darkened and discolored by various impurities. Some processes even produce aryltriazole products that are black. The free aryltriazole, that is an aryltriazole that is not in its anionic form, is insoluble in aqueous solutions and is often isolated from aqueous solutions as a solid or a melt. Such solids or melts usually contain most of the colorizing impurities initially in the aqueous solution. Such impurities are typically generated during the reaction processes that convert o-aryldiamines to aryltriazoles. Such impurities may also be introduced into the product aryltriazoles as impurities in the o-aryldiamine starting materials. The dark and colored impurities have been described variously as “colored-bodies” or “tars”. The exact nature of the colored impurities has been long conjectured, and likely include monoarylamines, monoaryldiazonium compounds, dimers of meta-aryldiamines, etc. The exact nature is of little consequence as the colored impurity bulk likely consists of many or all these components. References to the purification of aryltriazoles are U.S. Pat. No. 3,334,054 (1967), U.S. Pat. No. 3,564,001 (1971), U.S. Pat. No. 3,639,431 (1972), U.S. Pat. No. 3,970,667 (1976), U.S. Pat. No. 4,170,521 (1979), U.S. Pat. No. 4,269,987 (1981), JP 56-016478 (1981), U.S. Pat. No. 4,269,987 (1981), EP0303772 (1988), U.S. Pat. No. 4,918,195 (1990), JP04-360879 (1992), CN1,821,232 (2006), and JP224,014 (2007).
Specific methods of purification that have been disclosed include various distillation methodologies. Of consequence is that the colored impurities exhibit very similar boiling points, making separation by distillation difficult. Some components of the colored impurities boil at slightly lower temperatures than the target aryltriazoles while the remainder boil at higher temperatures, making fractionation necessary. Some components of the colored impurities may actually azeotrope with the target aryltriazole, making complete separation by a single distillation impossible. Also, colored impurities may entrain with the distillate stream, carrying droplets of higher boiling colored impurity to the overheads and contaminating the final product. Generally, because of the high boiling point of the aryltriazoles, high-vacuum distillation is necessary to prevent decomposition of the aryltriazole and to keep temperatures within safe ranges. In some cases, additives or special conditions are used. U.S. Pat. No. 4,918,195 and EP0308772 teach that distillation from basic compounds, i.e. NaOH, reduces the color of the distillate to a yellow which over time changes to grey. Similarly, U.S. Pat. No. 4,170,521 discloses the distillation of aryltriazoles from formaldehyde to generate benzotriazole and tolyltriazole of reduced color. In some cases, the aryltriazole is co-distilled or azeotroped with another compound to ease the distillation and improve yield. For example, U.S. Pat. No. 3,639,431 discloses the co-distillion of benzotriazole with polyethylene glycols. The distillate is an aryltriazole/polyethylene glycol mixture intended to be marketed as the mixture. Distillation has the disadvantage of needing expensive equipment. Another disadvantage is the concentration of high energy by-products in the still bottoms and the accompanying safety risks.
Another method of purification of aryltriazoles from colored impurities is adsorbance onto decolorizing media. Activated carbon, kieselguhr, diatomacious earth, clays, alumina, fuller's earth, pumice and sodium dithionite may be used to remove colored impurities from solutions of aryltriazoles. In many cases, the aryltriazole was in the anionic form, or was converted into the anionic form through the action of base, commonly hydroxide. For example, U.S. Pat. No. 3,970,667 discloses the use of active carbon, kieselguhr and then sodium dithionite sequentially to purify a sodium tolyltriazole solution. Once the anionic form of the aryltriazole is sufficiently purified, it is generally recovered by acidifying the solution to a pH of about 5 or 6 wherein the free aryltriazole is regenerated and separates from solution as the solid or melt, depending on temperature. U.S. Pat. No. 3,334,054 discloses a decolorization process in glycol solutions rather than aqueous solutions. JP04-360879 discloses performing the decolorization first under basic (alkaline) conditions, then repeats under acidic conditions before neutralizing and isolating the product. Unfortunately, the aryltriazole is also adsorbed onto the decolorizing media to some extent. As large quantities of decolorizing media are generally required to obtain acceptably purified aryltriazole, losses of aryltriazole to adsorption onto the media are appreciable. Also, large quantities of adsorbant must be used and subsequently disposed. In U.S. Pat. No. 3,970,667, 75 weight percent carbon plus 10 weight percent kieselguhr plus 10 weight percent sodium dithionite is used to effect decolorization. All these decolorization processes have the distinct disadvantage of handling solid decolorizing reagent, making addition and removal cumbersome.
Another method of purification of aryltriazoles from colored impurities is crystallization. For example, benzotriazole and/or tolyltriazole have been crystallized from alcohols, benzene, cyclohexane, and xylenes. However, U.S. Pat. No. 3,637,514 and U.S. Pat. No. 3,732,239 (same inventors) teach that there are colored contaminants in the aryltriazoles which cannot be removed by crystallization of the free aryltriazole. Crystallization is also inherently a low yield process. Though aryltriazoles are poorly soluble in cold and ambient temperature water, they have some solubility in hot water. Unfortunately, the colored impurities share the same property. Attempts to recrystallize free aryltriazoles from aqueous media typically precipitate the product with the colored impurities included. Generally, the aryltriazole separates as a melt, rather than a solid, and again, includes the colored impurities with it. The aryltriazole may be rendered soluble at ambient temperature by conversion to the aryltriazole anion with aqueous base. In this case, the colored impurities are also solubilized, generating a dark aqueous solution. Neutralization to a pH of about 5 or 6 regenerates the free aryltriazole, which precipitates as a solid or an oil. Without fail, the colored impurities are included in the precipitate, regenerating the discolored aryltriazole separated from a nearly colorless aqueous solution.
In the prior literature, aryl triazoles have been manipulated as the free aryltriazole under neutral conditions or as the aryltriazole anion under basic (alkaline) conditions. It has now been found that purification of aryltriazoles can be readily achieved by converting the aryltriazole to its cationic form under acidic conditions and separating the soluble aryltriazolium acid salt from the insoluble dark impurities by simple liquid/liquid phase separation.